If I make a copy of
Yes, you can do this, and it shouldn't cause any problems.
Everything that Git needs to keep track of your version history resides in the
.git folder. Thus, replacing that folder with an older version of that folder will roll back your repo to that older state. Unlike doing a Git rollback with
git checkout or
git revert, you will also lose all history that has been added to the repo since then.
To be safe, as @Danh suggested, you should delete the existing version of the folder before you restore the backup. (Actually, since Git objects are immutable, I'm not sure if this will be necessary. It shouldn't hurt though.) If you don't want to risk losing data from the existing version of the folder, you can also move or rename it and then restore the backup.
Depending on what you mean by "all fouled up", it may still be possible to recover without restoring from backup (especially if you haven't run any "dangerous" commands, such as anything with
--force). Feel free to search around or ask here if you need help!